Electric arc discharge devices



April 4, 1961 H. DE BOYNE KNIGHT 2,978,604

ELECTRIC ARC DISCHARGE DEVICES Filed Nov. 18, 1959 w //v Vf/VTOR f/[A/FY .05 50% HTTOR/VEY ELECTRIC ARC DISCHARGE DEVICES Henry de Boyne Knight, Rugby, England, assignor to The British Thomson-Houston Company Limited, London, England, a company of Great Britain Filed Nov. 18, 1959, Ser. No. 853,804

Claims priority, application Great Britain Nov. 18, 1958 7 Claims. (Cl. 313-193) This invention relates to electric discharge devices of the kind having a thermionic cathode and in which the discharge takes place in an ionizable medium at a low pressure, and possesses the characteristic of an arc. The

ionizable medium may consist of a gas or vapour, or a' mixture of gas and vapour. Mercury vapour constitutes a preferred ionizable medium.

The discharge may be uncontrolled, in which case the device is basically a rectifier; or it may be controlled by a control electrode when the device is similar to a thyratron.

In devices of this character the arc may in some circumstances transfer from the thermionic cathode to one or more spots, normally unheated, from which emission is of the cold-cathode type and in which, it is believed field emission plays a very much greater part than thermionic emission. The transfer may take place when the thermionic emission available is insufiicient to support.

the arc current corresponding to the voltage and impedance of the external circuit; in this case, the arc-voltage drop between cathode and anode tends to be high by comparison with that observed under the most favourable conditions. When such transfers take place the emission of the cold-cathode type is concentrated at one or more spots at which the current density is much higher than at the purely thermionically emitting cathode.

The junction between a conductor and an insulator has been found to provide suitable conditions for the formation of a cold-emission region. It it believed that an emitting spot is formed on the conductor at the junction due to the electric field set up by the charging of the insulator.

An object of the invention is to increase the current carrying capacity of the electric discharge device of the kind set forth by removing the limitation provided by the normal thermionic emission from acathode.

A further object of the invention is to provide a conductor-insulator junction means to which an arc initially established to an auxiliary cathode can transfer by utilising the formation of an emitting spot at the junction.

A still further object is to provide in the vicinity of the, normal arc path between a thermionic cathode and an anode means at which are discharge, initially established at the thermionic cathode, can root then relieving the thermionic cathode of the duty of carrying the main discharge current of the device.

A further object is to improve the current-carrying capacity of arc discharge devices by the provision of insulating material in contact with a conductor in the vicinity of the arc discharge path to encourage the transfer of the are from the cathode to the conductor.

In carrying out the present invention, insulating material positioned with relation to a conductor so as to encourage the formation on the conductor of a spot or spots on which the arc can root and to which the arc may transfer after its initiation to the thermionic cathode is provided in the vicinity of the normal path of the are ber 2,978,604 Patented Apr. 4, 19 61 mounted on a separate conductor led through the wall or the envelope of the device in a manner such that it is insulated from the leads to the other electrodes of the device. As a further alternative, the material may be mounted upon, or connected with, an electrode other than the thermionic cathode or the anode.

The insulating material is preferably of a refractory character, e.g. alumina.

In the accompanying drawings,

Fig. 1 shows in cross-section an electric discharge device in accordance with the invention,

Fig. 2 shows a modification of Fig. 1,

Fig. 3 is a detail view of a modified electrode construction, and

Fig. 4 is a view, similar to Fig. 1, of a further construction.

In the embodiment illustrated in Fig. l, a gas discharge tube comprises an envelope 1 containing an anode 2 and a directly-heated thermionically-emitting cathode 3 which is heated by passage of current from a separate voltage source. A control electrode, which is optional, is shown in the form of a perforated baffle 5 fitted transversely within a metal cylinder 4 surrounding the arc path between cathode and anode. The electron emitting surface of the cathode 3 may be coated with a material of low work function, such as an oxide of barium or strontium or a mixture of both of these formed by reduction of the carbonates of barium or strontium or a mixture of both of these carbonates during the processing of the device.

Close to the emitting surface of cathode 3 is a con,- ductor 7, which may, for example, be of nickel wire, to which, and in contact with it, is fitted an insulating member 6 to encourage the formation of an arc root on the conductor 7, the insulating member being thus positioned in the vicinity of the arc path between cathode and,

anode. The member 6 may for example be a length of tube of ceramic material, such as sinterecl alumina, threaded on to the conductor 7.

The modus operandi is as follows:

The discharge current begins as an are between the anode 2 and the hot cathode 3; as the current increases a point is reached at which the arc transfers to one or more points on conductor 7 close to.one or more of the points of contact between insulating member 6 and conductor 7 and current flows via the lead 7. The current to the hot cathode 3 may be limited and the transfer to 7 encouraged by the connection of an impedance 8 between the two members, either inside or outside the device.

In the alternative embodiment shown in Fig. 2, the insulating member is associated directly with the hotcathode 3. g In this example, the thermionic cathode 3 is a helix of suitable wire, coated with electron emissive material; the insulator is a length of ceramic tube threaded over one of the leads to cathode 3 and held in position near the emissive surface by means of metal eyelet 10 threaded on and welded to the lead. As in the arrangement shown in Fig. 1, the arc transfers to a spot Similar provisions may be made when the thermionic cathode is indirectly heated. Fig. 3 illustrates two embodiments utilising an indirectly heated cathode. The emitting surfaces of the indirectly heated cathode which may be coated with appropriate material of low work function, are shown as vanes 11 which project radially from a hollow metal cylinder 12, within which is a heater 13 which, when supplied with current from a separate source, raises the temperature of vanes 11 as required for electron emission. One or more cylindrical heat shields 14 a. closure end washer 15 and a perforated end washer 16 are fitted to increase the thermal eificiency of the cathode. The are discharge passes from the anode to vanes 11, passing through the aperture in washer 16. The current is conducted away by leads connected to 11 via the heat shields.

According to this embodiment of the invention an insulating member 17 is held in contact with a metal member 18 in close proximity to the emitting surfaces; for example, 17 may be a length of ceramic'tube threaded onto a nickel wire 18 which is welded to the cylinder 12.

Another embodiment illustrated in Fig. 3 is that of fitting an insulating member 19, such as a short hollow cylinder in the form of an annular washer of ceramic material, in contact with metal washer 16; it may, for example, be held in position by a second metal washer 20. Annular washer 19 may be used in addition to insulating member 16, or in place thereof.

In the device illustrated in Fig. 4, the cathode is of the form in which a metal plate 21 has on its surface a layer of electron emitting material 24 and is heated by a separate heater 23 enclosed within a heat shield cylinder 22. The layer 24 may be a loosely bound .layer comprising the carbonates (or the oxides after processing) of one or more of the alkaline earth metals with a proportion of finely divided metal, such as nickel, if desired, the mixture being applied as by spraying or painting. Alternatively 24 may be a layer of these materials sintered together and to 21, substantially as described by McNair, Lynch and Hannay in an article entitled Mold- A ed Thermionic Cathodes, Journal of Applied Physics,

October 1953, page 1335.

An insulating member 25 is held in contact with the base 21 close to the surface 24 for the purpose already described. Insulating member 25 may conveniently be, as shown, a length of ceramic tube fitted to 21, which may be shaped suitably as illustrated. If desired, 25 may also serve to space the cathode 21 from some other part of the electrode structure, such as the perforated grid baflfle 26 which is fitted transversely in a metal grid cylinder 4. If the insulator 25 is required to perform the latter function, it may be desirable to shape it or to shield a part of its surface or by both shaping and shielding it in this manner, to avoid its surface becoming continuously conducting due to material sputtered on to it by the action of the discharge. In the functioning of the device, the are discharge begins on the path from the anode 1 to the surface 24; but transfers to one or more spots on 24 or 21 near the line of contact with 25.

As a further alternative, the layer 24 may contain finely divided particles of insulation material such as powdered aluminium oxide, dispersed throughout the mixture. When the layer is sintered, the mixture may contain, for example, 70% by weight of nickel powder, of the carbonates of one or more of the elements barium, strontium and calcium, and 10% of aluminium oxide powder. In this case the arc discharge, initially supported by thermionic emission from the layer 24, transfers to one or more spots at which insulation and metal powders are in contact, and from which cold-cathode emission takes place. The use of the insulator as illustrated by may, if desired, be combined with the inc1usion of insulating particles in the layer 24.

What I claim is:

1. Means for increasing the current carrying capacity .4 of an electric arc discharge device comprising a sealed envelope containing an anode, and a thermionic cathode, said envelope being filled with an ionizable medium at a pressure such that a discharge between said anode and said cathode is of an arc-like character, said means comprising a conductor located in the vicinity of the discharge path between said anode and said cathode, and insulating material in contact with said conductor, said insulating material possessing the ability to encourage the formation on said conductor of a spot to which said are transfers from said cathode. 1

2. Means for increasing the current carrying capacity of an electric arc discharge device comprising an anode and a cathode between which, during operation, a discharge of arc-like character occurs, said cathode being supported by a conductor located in the vicinity of the arc path between said cathode and said anode, said means comprising an insulating member mounted on said conductor, the insulating material of which said insulating member is constituted being such as to encourage the transfer of the root of an are from said cathode to said conductor.

3. Means for increasing the current carrying capacity of an electric arc discharge device comprising an anode and a cathode between which, during operation, a discharge of arc-like character occurs, said cathode being supported by a conductor extending into the vicinity of the arc path between said cathode and said anode, said means comprising an insulating member mounted on said conductor, the insulating material of which said insulating member is constituted being such as to encourage the transfer of the root of an are from said cathode to said conductor.

4. An electric arc discharge device comprising an anode and an indirectly heated thermionic cathode between which, during operation, a discharge of arc-like character occurs, a discharge control electrode surrounding the arc path between said cathode and said anode, said control electrode providing an apertured barrier member through which said are passes, means for increasing the current carrying capacity of said device comprising insulating means spacing said cathode and said control electrode to provide at least one conductor-insulator junction, said insulating means surrounding the arc path between said cathode and said anode so that said junction is 10- ated in the vicinity of said are path to encourage the formation of a spot to which said are may transfer.

5. An electric arc discharge device as claimed in claim 4, in which said cathode has an emitting surface constituted of insulating particles, conducting particles, and particles of low work function in intimate contact to encourage the formation of cold emission from said emitting surface.

'6. An electric arc discharge device comprising an anode and a heat shielded thermionic cathode between which, during operation, a discharge of arc-like character occurs, said heat shielded cathodehaving an aperture through which said discharge passes to reach said anode, and a perforated insulating member surrounding said aperture and in intimate contact with a metal part in which said aperture is formed so as to be located in the vicinity of said discharge.

7. An electric arc discharge device comprising an anode and an indirectly heated thermionic cathode between which, during operation, a discharge of arc-like character occurs, said cathode having an emitting region constituted of insulating particles, conducting particles and particles of low work function in intimate contact with one another.

References Cited in the file of this patent UNITED STATES PATENTS 

